Method and device for coding electronic labels

ABSTRACT

A method for coding remotely detectable labels (10), whereby at least two elements (18, 19), the characteristics of which being changed by an external magnetic field, are arranged to form a label (10), the label (10) is exposed to a biasing magnetic field covering an interrogation volume (11), that is larger than the label, and the resonance frequency of the elements (18, 19), which is changed by the magnetizing field strength (H) of the magnetic field, is detected. The elements on each label (10) are oriented in predetermined angular relations with respect to each other so as to provide an identity for the label (10) determined by the angular relations, and the elements are exposed to a sequence of different field conditions. All possible combinations of as many magnetic field components as there are elements are compiled, and the magnetic field components are arranged in possible angular relations in the label (10). All angular relations or codes, which may be correct for different element combinations, are determined, and the determination is repeated until only one code for each unique element combination remains.

FIELD OF THE INVENTION

For remote detection of passive and simple elements different types ofmagneto-mechanical elements may be used, the magnetic and mechanicalproperties of which being influenced by an external magnetic field, aso-called bias field.

The elements are forced into states of mechanical self-oscillation, theelements thereby emitting and/or affecting a magnetic field in adetectable way.

DESCRIPTION OF THE PRIOR ART

Elements, which exhibit a comparatively large magneto-mechanicalcoupling, are suitable. The resonance frequency of these elementschanges due to the so-called Δ-E effect with the magnetic flux intensityalong the main direction of the element.

WO 93/14478 discloses and describes a method and a device for coding anddetection of remotely detectable gauges. A number of magnetic elementsof an amorphous material are placed upon each other on a label with acertain angular displacement, the angular position thereby constitutingone "bit" or code position. According to the coding method the anglesbetween the amorphous elements in the label are used to define the codeposition. Accordingly, each element may be arranged to represent anycode value within a predetermined number range. The expression "label"mentioned above generally refers to information carriers in the form ofseveral magnetic elements.

The method is based upon the use of at least one reference element,which is used to determine the angular positions of the rest of theelements. Thus, the effective range of actual codes is decreased.

BRIEF SUMMARY OF THE INVENTION

An object with the present invention is to provide a method and a devicefor coding and detection of magnetic elements, where all of the elementsare used as coding elements. Further objects and advantages are obviousfrom the following description and claim.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by the following embodiments with referenceto the accompanying drawings, on which

FIG. 1 is a schematic perspective view of an arrangement for carryingout the method according to the invention, and

FIG. 2 is a graph illustrating the method for detection according to theinvention.

DETAILED DESCRIPTION

In a preferred embodiment two excitation means 12A and 12B are connectedto an electronic controller 15 via two driver and amplifier units 16Aand 16B. The excitation means 12A and 12B are formed as magnetic coils,which are parallelly and coaxially arranged with respect to each other.By this an interrogation zone 11 is obtained between the excitationcoils with a symmetry plane with respect to the excitation fieldgenerated by the excitation coils. With equal signal strength from bothof the excitation coils the symmetry plane is obtained at half thedistance between the coils. Two detection means, preferably magneticcoils, are symmetrically arranged with one means at each side of thesymmetry plane. The detection coils 13A and 13B are operativelyconnected to a central electronic controller 15. In the embodiment shownthe detection coils 13A and 13B are connected to amplifier units 17A and17B, respectively, which in turn are connected to the controller 15.

When the arrangement according to the figure is used, identicalexcitation signals are generated by the controller 15 in both theexcitation coils 12A and 12B. The signals from both the detection coils13A and 13B are amplified in the amplifiers 17A and 17B, and a signalcorresponding to the difference between the recorded signals in thedetection coils is determined in the central controller 15. If all thesignals are generated fully symmetrically, the detected signal withoutthe presence of any resonance elements will have a zero value, since thesame signal is recorded in both the detection coils 13A and 13B.

The symmetry required does not necessarily have to be geometrical. It isalso possible to accomplish the desired symmetry electronically. Inorder to compensate for errors and defects in the manufacture and/orassembly process, displacements and the like, or other sources of a lostsymmetry in the excitation signal to the detection coils, it isfurthermore possible to use active balancing. Such an active balancingmay easily be accomplished, if the signal to each of the excitationcoils is controlled individually. At chosen instants the level of theexcitation signal in the difference signal from the detection coils maybe monitored, and the excitation control may be adjusted, so that thedifference signal with respect to the excitation signal will become zeroagain.

It is also possible to directly adjust the difference signal to balancewith the use of, e.g., differential amplifiers at the detection side.Also other realizations with different coil configurations for theexcitation and detection coils are possible within the scope of theinvention.

Several magnetic elements 18, 19 are arranged with certain angulardisplacements with respect to each other on a label 10, which is placedin the interrogation zone 11. Possible angles between the elementsconstitute code values, and a certain set of angles constitutes anidentification for the label. The angles are selected from apredetermined set of allowed angles.

When a label provided with an unknown combination of angles is going tobe detected, the following steps are taken. The elements on the labelare forced into self-oscillation, the resonance frequencies of theelements thereby being detectable. The resonance frequency of an elementor elements is a function of the exposing magnetic field, or themagnetizing field strength, whereby the magnetic field exposing eachelement may be determined. It should be noted, that the magnetic fieldis a projection of the actual magnetic field on the element lengthextension and therefore constitutes a component of the actual magneticfield.

Then all possible combinations of the magnetic field components are puttogether in a calculating unit 20. Possible combinations are determinedby the predetermined sets of angles. Every component pair is usedaccording to FIG. 2 to form a possible, actual magnetic field vector.Every possible combination gives rise to several magnetic field vectors,and taken together the number of possible magnetic field vectors becomeslarge.

In other words all possible difference angles between all code elementsin a label are computed and matched against each other with respect tothe projecting magnetizing field strength H. According to FIG. 2 eachpair gives rise to a computed H-vector:

    H.sub.0 -H.sub.1, H.sub.1 -H.sub.2, H.sub.2 -H.sub.3, H.sub.0 -H.sub.2, H.sub.1 -H.sub.3, and H.sub.0 -H.sub.3.

Each element direction, or actually the H-vector in the elementdirection, may be expressed according to the following:

    H.sub.1 =H cos x

    H.sub.0 =H cos (α-x),

where

α=the angle between the elements (and between the H-vectors projected onthe elements); the a value is included in a set of angular values.

All elements in a label are exposed to the same H-vector, andconsequently every correct code combination between all the determinedones should give the same H-vector value. Every combination of codes orangles, that gives the same H-vector, therefore may constitute a reallabel combination.

The method described above is then repeated with different magneticfield conditions so as to filter out any incorrect (i.e. not real)combinations. Every new situation (gradient or direction) impliesseveral conditions, which have to be fulfilled for the combination to becorrect. After a number of repetitions only the correct (real) codesremain, whereby all labels present in the interrogation zone, includingthe ones with identical sets of codes, are detected.

I claim:
 1. A method for detecting codes on remotely detectable labelsof the type carrying at least two elements having a length extension,the resonance frequency of an element depending on the strength of thecomponent of an external magnetic field along the length extension ofthe element, the elements on each label being oriented in predeterminedangular relations with respect to each other so as to provide a code forthe label determined by said angular relations, wherein:the labels areexposed to a sequence of different biasing magnetic fields covering aninterrogation volume that is larger than a label; the resonancefrequencies of the elements of labels in the interrogation volume beingdetected for every biasing magnetic field; the component of the biasingmagnetic field vector along the length extension of each element isdetermined from the corresponding detected resonance frequency; themethod comprising:(a) determining all possible combinations of angularrelations between pairs of elements of a label; (b) for a combination,computing resulting magnetic vectors from different pairs of saiddetermined components of a particular biasing magnetic field, using theangular relations of the combination; (c) eliminating any combinationnot having the same resulting magnetic vectors computed for differentpairs of elements; and (d) repeating steps (b) and (c), using differentbiasing magnetic fields, until only such combinations remain thatcorrespond to actual codes of labels within the interrogation volume.